Cold Dark Matter

is it simply because cold dark matter is slow moving (non-relativistic) and slow moving things are more likely to clump together to form larger things because they possess less energy and momentum? or is there more to it?

I can't answer your question specifically. However, the various studies of cold vs. warm are made using computer simulations of how the universe would evolve after the big bang and comparing to what is observed.

This is due to damping of density perturbations. For collisionless particles the most important damping effect is the free streaming damping. Free streaming damping means that the gravitation of the density perturbations of those particles cannot bind them below their free streaming length, and therefore density perturbations smaller than this length cannot form. The free streaming length is the lenght that those particles can travel until they become non-relativistic. Heavy dark matter particles like CDM particles are assumed to become non-relativistic very early, having thus a small free streaming length. This, in turn, means that density perturbations on small scales can be formed leading to a bottom-up scenario of structure formation.

You can find a small introduction in section 7 of the reference 21 (Neutrinos and structure formation in the universe) here:
http://www.ita.uni-heidelberg.de/~msb/Publications/pubConferences.html [Broken]